Selective laser melting (SLM) and selective laser sintering (SLS) apparatus produce objects through layer-by-layer solidification of a material, such as a metal powder material, using a high energy beam, such as a laser beam. A powder layer is formed across a powder bed in a build chamber by depositing a heap of powder adjacent to the powder bed and spreading the heap of powder with a wiper across (from one side to another side of) the powder bed to form the layer. A laser beam is then scanned across portions of the powder layer that correspond to a cross-section of the object being constructed. The laser beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified, as required. An example of such a device is disclosed in U.S. Pat. No. 6,042,774.
The process is carried out in an inert gas atmosphere because the metal powder is highly reactive with gases, such as oxygen. As described in International patent application No: 2010/007394, it is known to first form a vacuum in a build chamber and then refill the chamber with an inert gas in order to ensure low oxygen content in the resultant atmosphere. For example, using this technique, an oxygen content of the atmosphere in the chamber may be reduced as low as 1000 ppm. Over the course of the build, the oxygen content may drop further due to the remaining oxygen in the chamber being consumed as oxides in the part being formed. The amount of oxygen absorbed will depend on the material being used. For example, titanium is much more able to absorb oxygen than steel or aluminium. It is desirable to have a consistent oxygen content in the atmosphere throughout the build in order to achieve consistent build properties throughout the part.
It is believed that moisture in the air and the powder leads to hydrogen being absorbed into the material of the part, which results in hydrogen gas porosity in the part.
The presence of nitrogen in the inert atmosphere during a build may also be undesirable.